1. Field of the Invention
The present invention generally relates to a technology of depositing a film having a film thickness different at the top and bottom of a patterned surface of a substrate.
2. Description of the Related Art
Spacer defined double patterning (SDDP) technique has been developed for photolithography to enhance the feature density. The resolution of a photoresist pattern begins to blur at around 45 nm half-pitch. Thus, for the semiconductor industry, double patterning appears to be the only lithography technique to be used for nodes smaller than the 45 nm half-pitch nodes.
Conventionally, SDDP was performed as illustrated in FIG. 3. First, pre-patterned features 31 (e.g., photoresist) are formed on a hard mask 32 as shown in FIG. 3(a). Next, a conformal spacer film 33 is deposited to cover the pre-patterned features 31 and the hard mask 32 as shown in FIG. 3(b). A spacer will be a film layer formed on a sidewall of the pre-patterned feature 31. In order to form a spacer, anisotropic spacer etching is conducted as shown in FIG. 3(c) to remove all the film material on the bottom surface and the top surface, i.e., all the horizontal surfaces, leaving only the material on the non-horizontal surfaces including the sidewalls (33a and 33b). By removing the original pre-patterned features 31, only the spacer is left on the hard mask 32 as shown in FIG. 3(d). However, since the top portion of each pre-patterned feature is not flat, but at least partially rounded as shown in FIG. 3(a), not only the material on the sidewall but also the material 33a on a tapered or rounded portion (non-horizontal portion) of the pre-patterned feature is left when the anisotropic spacer etching is complete, forming an overhang portion 33c (inwardly declining portion) when the removal of the original pre-patterned feature is complete as shown in FIG. 3(d).
The above problem can be illustrated in FIGS. 6A and 6B. If a photoresist (PR) pre-patterned feature 63 formed on a substrate 62 has ideally a flat top, a conformal spacer film 61 covers the pre-patterned features and the bottom surface evenly as shown in the top figure of FIG. 6A. By using anisotropic spacer etching to remove the material on the horizontal surfaces (the middle figure of FIG. 6A), and by removing the original pre-patterned PR, a vertical spacer can be formed as shown in the bottom of FIG. 6A. However, forming the pre-patterned feature having a flat top is extremely difficult and in reality, all pre-pattered features have at least a partially rounded top. The shape of the pre-patterned feature illustrated in FIG. 3 is simplified for easier understanding and the actual shape of the pre-patterned feature is likely to be like the one illustrated in FIG. 6B. If a photoresist (PR) pre-pattered feature 67 formed on a substrate 66 has a rounded top, a conformal spacer film 65 covers the pre-patterned features and the bottom surface evenly as shown in the top figure of FIG. 6B. By using anisotropic spacer etching to remove the material on the horizontal surfaces (the middle figure of FIG. 6B), followed by removing the original pre-patterned PR, not only a vertical portion formed on the sidewall but also a curved portion formed on the rounded top is left as shown in the bottom figure of FIG. 6B. In the above, if the etching continues until the overhang portion is removed, the substrate is over-etched and the surface layer such as a hard mask is undercut. As a result, unavoidably the overhang portion of the spacer is formed, and due to the overhang portion, the distance 69 between the top edges of the spacer becomes smaller than that between inner vertical portions of the spacer.
Returning to FIG. 3(d), using the spacer 33a, 33b, anisotropic etching is performed for pattern transfer as shown in FIG. 3(e), wherein a pattern transfer layer 34 is formed on a substrate 35. Because of the overhang portion 33c of the spacer, the distance D1 between the top edges of the spacer is smaller than the distance between the vertical portions of the spacer (D1+2D2), and the pattern transfer layer 34 is not accurately etched in the vertical direction. That is, at an upper portion of the hard mask and the pattern transfer layer, unexpected critical dimension (CD) changes occur, and at the bottom, CD increases by D2 by the overhang residual portion as shown in FIG. 3(f).
To solve the above problem, the use of an additional hardmask layer having high etch selectivity (e.g., amorphous CVD carbon or spin-on carbon) may be adopted. However, that requires more processing steps and processing complexity, thereby increasing production cost.